"Michelle and I are shocked and saddened by the horrific and tragic shooting in Colorado. Federal and local law enforcement are still responding, and my Administration will do everything that we can to support the people of Aurora in this extraordinarily difficult time. We are committed to bringing whoever was responsible to justice, ensuring the safety of our people, and caring for those who have been wounded. As we do when confronted by moments of darkness and challenge, we must now come together as one American family. All of us must have the people of Aurora in our thoughts and prayers as they confront the loss of family, friends, and neighbors, and we must stand together with them in the challenging hours and days to come."

Also from the campaign trail came comments from presumptive GOP nominee Mitt Romney, who cited preliminary casualty reports:

"Ann and I are deeply saddened by the news of the senseless violence that took the lives of 15 people in Colorado and injured dozens more. We are praying for the families and loved ones of the victims during this time of deep shock and immense grief. We expect that the person responsible for this terrible crime will be quickly brought to justice."

"We share the shock and sadness of everyone in the motion picture community at the news of this terrible event. We extend our prayers and deepest sympathies to the victims, their loved ones and all those affected by this tragedy."

Reactions on Twitter are those of sustained shock and grief, perhaps best characterized by Michael Moore, whose 2002 documentary Bowling For Columbine explored the issue of gun violence in America (particularly in Colorado):